1. Hemagglutinability of some strains of influenza virus, newly isolated by the chick embryo inoculation with mouth washings from patients, was inhibited in the first few passages not only by the immune sera of homologous type but also by the immune sera of heterologous type of influenza virus and of immunologically indifferent virus, for example mumps virus.
2. This property was found in the strains of both types of influenza virus A and B.
3. This property disappeared gradually as the passages proceeded further.
4. The appearance of this property is independent of titer or kinds of materials-allantoic or amniotic fluids-used as hemagglutinin, and dilution of inoculum. The difference in the property is observed between the allantoic fluids harvested from each chick embryos inoculated simultaneously with the same materials, and also between the sera used in the test.
5. The difference, observed between the allantoic fluids harvested from each chick embryos at the same passage, disappears after several chick embryo passages when each line of virus is separately passed.
6. The newly isolated viruses are divided into two groups by the grade of inhibition by homologous immune sera in the course of losing the property; the titer of serum increases gradually to one of them and it does not to the other.
7. The immunogenicity of the virus showing the property is stable; the serum of chicken immunized with this virus reacts specifically with the hemagglutinin in the hemagglutination inhibition test.
8. The reconvalescent serum of patients, from whom the virus showing the prorerty is isolated, also reacts specifically with the hemaggglutinin in the test.
9. It is discussed and concluded that the property is one of the biological characters of influenza virus showing in the course of adaptation to the chick embryo.

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A new method of egg culture has been devised. The content of an fertilized chicken egg, whose shell is totally removed by breaking, can servive in the incubator at least for fourteen days, showing nearly the same growth as in the natural state. This naked chick embryo in porcelain cup with glass cover appears to provide a hopeful method for the cultivation of viruses because of its facility of handling as well as observations.

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